Energy exploration and exploitation using boreholes drilled into earth formations require the monitoring and evaluation of physical conditions, such as the resistivity or conductivity of the earth formations around a single borehole, often up to a radial distance of several hundred meters from the borehole, or in the space between two boreholes which are separated by a distance of several hundred meters or more. An example of the above is the current conventional reservoir monitoring using cross-well tomography. However, traditional logging techniques typically do not permit the radial investigation of the earth formations surrounding a single borehole up to distances exceeding 2-3 meters at best. Mining operations have utilized transient electromagnetic measurement techniques for resistivity/conductivity in which a large surface dipole antenna (often several hundred meters in length) is utilized with a transient electromagnetic receiver located in a borehole drilled in the earth to make measurements in vertical and horizontal zones in the earth surrounding the borehole and between the borehole and the earth's surface.
There are generally two possible ways to excite electromagnetic fields: frequency domain excitation (including direct currents DC! in the zero frequency limit), and time domain excitation to generate transients. Frequency domain excitation is based on the transmission of a continuous wave of a fixed (or sometimes even mixed) frequency and the measuring of the response at the same band of frequencies. Discrete frequencies are linked through the skin depth (Kaufman and Keller, 1983) to certain formation volumes. Time domain excitation employs a square wave signal (or pulses, triangular waveforms or pseudo random binary sequences) as a source and the broadband earth response is measured. When the transmitter current is switched abruptly, the signals appearing at a receiver caused by the induction currents in the formation by a transmitted signal are called transients, because the receiver signals start at a certain value and then decay (or increase) with time to a constant level.
One major problem of alternating current (AC) measurements in the frequency domain excitation is the strong coupling between transmitter and receiver, known as the direct mode. The direct mode arises from the magnetic field caused directly by the current in the transmitter loop. This phenomenon puts severe restrictions on the achievable accuracy of measurements and, as a result, on a range of measurable formation resistivities. A problem of direct current (DC) excitation (when frequency goes to zero) is that the measured signal is a composite signal comprising a mixture of contributions derived simultaneously from different regions. This will drastically deteriorate the possible practical resolution available by such methods. The commonly implemented ways to enhance resolution of frequency domain measurements are either to design multi-coil devices permanently focused on certain spatial areas of formation such as conventional borehole induction measurement tools, or to use an array of measurements and multi-target processing techniques to allow numerical focusing of the measurement onto a certain predetermined region in space such as using array-type induction or laterolog measurement tools. In both cases, the problem is that the resulting net signal is very small compared to the original total measured signal which means that a high accuracy and resolution of raw measurements is required. In addition, if multiple transmitter-receiver configurations are used, differential processing techniques can significantly improve vertical and radial resolution of the measurements.
In contrast, however, time domain signals are inherently not lumped, and spatial filtering of time domain (transient) data allows more direct computation and precise separation of component responses. The primary signal which does not contain information about the earth medium parameters and is very large is not included in the transient mode measurement. Moreover, a properly excited transient field (such as a switch-off impulse) does not have a direct mode problem, which means that the whole measured signal is characterized primarily by the features of the resistivity distribution in the surrounding space.
The ability to separate in time the response of different spatial areas is an important characteristic of the transient electromagnetic field. After switching off the transmitter current induced currents of the same geometry appear in the nearby area due to Faraday's law. Not being supported by extraneous forces, this current begins diffusion to the outer space. This diffusion is followed by attenuation and dispersion in which the spatial resolution in the later time stage becomes significantly reduced.
However, transient field data in the later time stages have proved to be more sensitive to the distant formation resistivity than frequency domain or DC-data (Kaufman and Keller, 1983); (Strack, 1992). What is known as geometric factor contribution in the frequency domain, based on Doll's approximation (Doll, 1949), does not participate at all in the latest time stage of the transient electromagnetic measurement technique. This gives a unique opportunity to combine both transient and frequency domain measurements to utilize the complementary information they contain.
Historically, measurements of formation characteristics in a borehole have been accomplished for very near-to-the-borehole radial distances. The electromagnetic noise in the borehole is much lower than at the surface because the earth acts as an exponential lowpass filter. Because of the small size of the borehole and the harsh environmental conditions, an electromagnetic measuring instrument is, as a practical matter, restricted in the amount of sophistication that can be reliably constructed in the instrument. To overcome such restrictions, the more easily implemented frequency domain electromagnetic measurement methods have been chosen as a basis for the measurement of formation characteristics using logging tools such as induction and laterolog tools.
Only recently (Tabarovsky et al., 1992) has it been possible to model reasonably realistic borehole transient electromagnetic situations. Parallel to such developments in the area of numerical modelling, the electronic capabilities in high power switching, amplifier design and data transmission have improved, thereby making a time domain borehole system feasible. The limitation on the radial depth or range of investigation in deep measuring transient electromagnetic investigation is determined primarily by the signal-to-noise level of the measurements, which are related to the available impulse energy and to the measurable signal levels.
The task of interpreting deep radial electromagnetic soundings in a formation of interest can be simplified if the structure of the boundaries is obtained or approximated from other geophysical data (gravity, seismic, borehole logs, geologic surveys, etc.). This additional information, for example, can be used to keep certain parts of the earth parameters fixed while interpreting for the others.
The prior art and the relationship between frequency (continuous wave) and transient electromagnetics is well described in two geophysical monographs by Kaufman and Keller (1983) and Strack (1992). The latter also described the required necessary improvement in hardware design for highly sensitive measurement (Rueter and Strack, 1991). For mining applications, the use of the transient electromagnetic method is fairly common and documented in numerous publications (i.e.: Crone, 1985; Thomas, 1987). Modeling of the transient response started fairly early (Lech, 1975) but has been restricted to a fairly simple and approximate models (Jarzyana, 1979; Chew et al., 1981; Eaton and Hohmann, 1984; Raiche and Bennett, 1987; Lee and Buselli, 1987; Thomas, 1987; West and Ward, 1988; Anderson and Chew, 1989, Liu and Shen, 1991). Only with the advent of new modeling codes (Tabarovsky et al.) could a more realistic investigation beyond the basic feasibility as described by Anderson and Chew (1989) be accomplished.